The sea breeze/land breeze diurnal circulation within the Los Angeles Basin and adjacent waters transports marine air into the basin during the day and urban air to Santa Monica Bay during the night. Nitryl chloride, ClNO_2 is a nocturnal trace gas formed frorn the heterogeheous reaction of dinitrogen pentaoxide (N_2O_5) with chloride containing aerosol. Its photolysis after sunrise produces atomic chlorine radicals and regenerates NO_2, both of which may increase ozone production. Mixing of the chloride source frorn marine sea salt with the urban NO_x source in Los Angeles provides conditions ideal for the production of ClNO_2. This paper presents an analysis using a wind profiler on the coast and measurements of ClNO_2 and its precursors made frorn both ship and aircraft to assess the prevailing meteorological conditions important for ClNO_2 production in this region, with a particular focus on the production over water within the land breeze phase of the circulation. A box model is used to calculate an upper limit to the amount of ClNO_2 capable of being produced strictly over Santa Monica Bay during the land breeze. On three out of the four nights of ClNO_2 measurements in Santa Monica Bay, the ClNO_2 exceeds the upper limit calculated using the box model and shows that the majority of the ClNO_2 is produced over the city and transported to Santa Monica Bay by the land breeze. This ClNO_2 transport suggests the sea breeze more efficiently transports aerosol chloride inland than land breeze transports NO_x offshore.
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